William Leonard Langer (March 16, 1896 – December 26, 1977) was the chairman of the history department at Harvard University. He was on leave during World War II as head of the Research and Analysis Branch of the Office of Strategic Services. He was a specialist on the diplomacy of the 1870-1900, and 1937-41 periods. He edited numerous books including a series on European history, a large-scale reference book, and a university textbook.

When William was only three years old, his father died unexpectedly, leaving the family in difficult circumstances. Nevertheless, his mother, who supported the family by working as a dressmaker, made education a priority for her children.

His job and education were interrupted by military service World War I. After the war, he returned to his studies and obtained his Ph.D. in 1923. In 1921 he married Susanne Katherina Langer (née Knauth) who became a noted philosopher. They had two sons together before divorcing in 1942.

He taught modern European history at Clark University for four years before accepting an assistant professorship at Harvard. In 1936 Langer became the first to hold the Archibald Coolidge chair.

Langer was remembered at Harvard especially for his "History 132" course on modern European history, "History 157" on the Ottoman Empire, and the graduate seminars held at his home.[8][9]

With the help of other scholars during the 1930s, Langer completely revised the Epitome of History by German Scholar Karl Ploetz. Langer’s massive work was published in 1940 under the title An Encyclopedia of World History.[10] Its fifth edition (1972) is the last to be edited by Langer. Peter N. Stearns and thirty other prominent historians edited the sixth edition, published in 2001. Stearns paid tribute to Langer's great achievement in the introduction to the new edition.

In 1957 Langer urged historians to expand their insights with techniques from modern psychology.[11]

^"CIA’s Directorate of Intelligence Marks its 50th Anniversary". Central Intelligence Agency. November 4, 2002. Retrieved 2014-02-03. William Langer Award for outstanding analytic contributions to the DI. Langer - a distinguished scholar and pioneer OSS analyst - was the first chairman of CIA’s Office of National Estimates and later served on the President’s Foreign Intelligence Advisory Board (PFIAB). During his career, Langer demonstrated the feasibility of performing intelligence analysis by combining information from multiple intelligence collection disciplines, including imagery, signals intercepts, and human intelligence.

^Waggoner, Walter H. (July 10, 1981). "WALTER LANGER IS DEAD AT 82; WROTE SECRET STUDY OF HITLER". New York Times. Walter Langer was born in Boston on Feb. 9, 1899, the son of Charles Rudolph and Johanna Rockenbach Langer. He was a practicing psychoanalyst from the late 1930's until about 1960, and he was the The brother of William L. Langer, the Harvard historian, and Rudolph Ernest Langer, chairman of the mathematics department at the University of Wisconsin.